Consequences of anthelmintic resistance on liveweight gain of lambs on commercial sheep farms

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AIM: To assess the consequences of anthelmintic resistance on commercial sheep farms on liveweight (LW) gains and susceptibility to diarrhoea of growing lambs. METHODS: Five farms with a history of resistance to benzimidazole drenches were selected. On each farm, 150 ewe lambs were randomly allocated to 1 of 3 treatment groups (n=50). Group 1 was treated 5 times at 28-day intervals with oxfendazole (to which nematode resistance had been demonstrated); Group 2 was treated 5 times at 28-day intervals with levamisole or levamisole + oxfendazole (to which nematode resistance had not been demonstrated) and; Group 3 was treated with a combination of slow-release albendazole capsules and oral moxidectin (intended to totally suppress nematode infection). Differentiated faecal egg counts (FECs), FEC reduction tests (FECRT), LW gain and degree of faecal soiling around the breech (dag scores) were determined and compared between groups. RESULTS: Pre-treatment FECs in Group 1 were similar to or slightly lower than those of Group 2. Efficacy of oxfendazole in Group 1 varied between farms and over time within farms, the reduction in FECs after treatment varying from 0 % to >95 %. The genera identified as resistant on each farm also varied over time. Lambs from Group 3 had lower FECs, lower dag scores and higher growth rates than lambs from Group 2, which in turn had higher LW gains than lambs from Group 1. The overall LW gains during the 5 months of the study were 7.03 ±0.27 kg for Group 1,8.33 ±0.27 kg for Group 2, and 9.91 ±0.31 kg for Group 3 (mean ±SEM; means differed significantly from each other, p<0.01). CONCLUSIONS: The decrease in growth rates associated with the continued use of an incompletely effective anthelmintic were surprisingly small in situations where all lambs grazed together.